A semiconductor laser typically has a body of semiconductor material having a thin active medium between cladding regions of opposite conductivity type which form electrodes. To increase the output power, a guide layer having a refractive index which is intermediate the active and cladding layers may be interposed between one of the cladding regions and the active region. Light generated in the active medium propagates in both the active and guide layers thereby forming a beam at the emitting facet of the material. The cavity region comprising an active medium or the combination of a guide layer and an active medium restricts oscillation in the transverse direction, the direction perpendicular to the plane of the layers, to the fundamental optical modes.
However, such devices produced several frequencies of output or modes which is hot preferred. To obtain a desired single-frequency oscillation, a frequency selective grating element can be integrated within the laser. In the distributed feedback (DFB) laser, the grating element is built in a waveguide layer adjacent to the active medium. In the conventional DFB semiconductor laser having a uniform diffraction grating, there are two longitudinal modes with equal threshold gain in principle on both sides of the Bragg wavelength. In practice, it is frequent that such DFB semiconductor lasers operate in double lasing modes, causing so-called mode-hopping noise. To overcome this defect and to effect the oscillation in a single longitudinal mode, DFB lasers with .pi./2 phase shifted grating structure have been made. To obtain a single-frequency laser, a .pi./2 phase shift can be incorporated into the grating element at the center of the laser cavity.
The physics which describes the lasing phenomena of the type described is well-known in the art and need not be further described here. An excellent discussion including the equations which generally describe semiconductor lasers is found in Lee, "Recent Advances in Long-Wavelength Semiconductor Lasers for Optical Fiber Communication," Proceedings of the IEEE, Vol. 79, No. 3, pgs. 253-276 (March 1991). This article is incorporated herein by reference.
Because most DFB lasers operate under a direct high frequency current modulation, a problem known as the spatial hole burning effect has to be considered. Spatial hole burning is a small localized active medium property change caused by localized mode pattern peaks. The local peaks result in a localized reduction of the number of occupied conduction-band states and empty valance-band states. Due to the finite mobility of the injected carrier in a semiconductor material, locally large optical fields that cause a locally large stimulated emission result in an uneven spatial carrier distribution. The uneven distribution changes the gain and refractive index profile of the active medium and causes a local Bragg frequency change. The spatial hole burning effect can destroy the single mode operation especially when the DFB laser is operated under a large current modulation.
The spatial hole burning effect also increases with the coupling strength kL of the DFB laser (k=coupling coefficient, L=total laser length). Modern optical communication systems require single mode, narrow spectral bandwidth lasers. Since the bandwidth is inversely proportional to the length of the laser, lengthening the laser narrows the bandwidth. However, the lengthening of a laser increases the coupling strength and hence the spatial hole burning effect. It has been found that spatial hole burning is the main effect which limits the possible length of a single mode DFB laser.
As an alternative to the use of a phase-shifted grating structure, a localized equivalent refractive index phase shift structure can be employed. The central shifting portion of the active medium generally has a different width than the end portions of the active medium and an equivalent phase shift thus can be achieved. However, due to the lack of understanding of the structure, the lasing condition was copied from a self-consistent oscillation used in a shifted grating structure. The length of the central portion was always chosen small in comparison with the total laser length. Thus, the central portion would be less than one-fifth or one-tenth of the total length of the active medium which concentrates the phase shift in a small area. As a result, such DFB lasers suffer a severe spatial hole burning effect especially when the coupling strength is over 3.
A discussion of the physics of the spatial hole burning effect including the mathematics describing such phenomena can be found in Kimura et al., "Coupled Phase-Shift Distributed-Feedback Semiconductor Lasers for Narrow Linewidth Operation," IEEE Journal of Quantum Electronics, Vol. 25, No. 4, pgs. 678-683 (April 1989). The cited article is incorporated herein by reference.
Therefore, what is needed is a DFB laser which can reduce the spatial hole burning effect while significantly maintaining a large gain margin for narrow linewidth, single mode operation. In addition, the structure should be designed to be larger than the pitch of feedback gratings and thus not put an extra requirement on lithography techniques used to fabricate these lasers.
The present invention meets these needs.